Astrarium: Judex
Astrarium: Draconis
Astrarium: Peraquialis
Astrarium Vault
After you solve all three astrariums in the Hinterlands, the door to the treasure chamber they locked will open. Its location is within the final cave you enter as part of the Apostates in the Witchwood side quest, and it will be marked on your quest map. See the information on that quest for further details on how to reach this cave.